Process for producing a thermoelectric converter

ABSTRACT

Method for fabricating a thermoelectric converter having a plurality of series-connected thermoelement cells, which are connected in series with one another by means of a plurality of first electrical conductor tracks ( 3 ) and each of which has a first body ( 4 ) made of thermoelectric material of a first conduction type and a second body ( 5 ) made of thermoelectric material of a second conduction type. The thermoelement cells are fabricated by means of method steps appertaining to semiconductor technology.

The invention relates to a method for fabricating a thermoelectricconverter.

DE 39 35 610 A1 describes a Peltier cooler having an interconnection ofn- and p-doped semiconductor segments which are connected together withthe aid of metal bridges. In this case, the metal bridges are applied onAl₂O₃ substrates and the semiconductor segments in the case of one ofthese substrates are vapor-deposited onto the metal bridges presentthere. Peltier coolers of this type have a physical size in thecentimeters range and cannot readily be miniaturized, with the resultthat the power density is very low. Furthermore, the Al₂O₃ substrateshave a very low thermal conductivity, which impairs the effectiveness ofsuch Peltier coolers.

The object of the present invention consists in developing a method forfabricating a thermoelectric converter of the type mentioned in theintroduction which allows a higher integration level of thethermoelement cells and thereby enables the fabrication ofthermoelectric components with a higher power density and in whichcost-effective processes are used.

This object is achieved by means of a method having the features ofclaim 1. Subclaims 2 to 10 relate to advantageous developments of themethod.

The method according to the invention is used to fabricate athermoelectric converter having a plurality of series-connectedthermoelement cells, which are connected in series with one another bymeans of a plurality of first electrical conductor tracks and each ofwhich has a first body made of thermoelectric material of a firstconduction type and a second body made of thermoelectric material of asecond conduction type, which are connected to one another by means of asecond electrical conductor track and which are arranged in asandwich-like manner between a first and a second substrate wafer whichis electrically insulating or has an electrically insulating layer.

In the method, the first electrical conductor tracks are formed on amain area of the first substrate wafer. The second electrical conductortracks are produced on a main area of the second substrate wafer. In thecase of at least one of the two substrate wafers, at least one layermade of thermoelectric material is applied on the same side on which theconductor tracks are produced. Said layer is patterned by means ofphotomask technology and etching in such a way that the first and secondbodies of the thermoelement cells are produced.

After the processing of the two substrate wafers, the latter are joinedtogether e.g. by means of thermocompression, soldering, adhesive bondingor anodic bonding to form a sandwich composite, in which the first andthe second bodies are arranged between the two substrate wafers and areconnected by means of the first and second electrical conductor tracksto form thermoelement cells connected in series.

In each case a first and a second body are connected on a first side bymeans of a first conductor track to form a thermoelement cell, which areconnected in series with one another by means of the second conductortracks on a second side—opposite to the first side—of the first andsecond bodies.

In a preferred embodiment for producing the first and second conductortracks and the first and second bodies, firstly a first electricallyconductive layer is applied to the main area of the first substratewafer. A layer made of thermoelectric material is subsequently depositedonto said electrically conductive layer and a plurality of doped regionsof the first conduction type and a plurality of doped regions of thesecond conduction type are then formed in said layer made ofthermoelectric material.

The layer made of thermoelectric material is subsequently patterned bymeans of photomask technology and etching to form first and secondbodies; i.e., after the patterning mutually isolated first and secondbodies remain on the first electrically conductive layer.

After this process step, the first electrically conductive layer ispatterned for example once again by means of photomask technology andetching to form first conductor tracks which each connect a first and asecond body to one another on one side of the bodies, thereby producinga plurality of mutually isolated thermoelement cells.

However, the first electrically conductive layer can also be patternedto form first conductor tracks even before the application of the layermade of thermoelectric material.

Before, during or after these steps, a second electrically conductivelayer is applied to the main area of the second substrate wafer and issubsequently patterned, for example, once again by means of photomasktechnology and etching to form second conductor tracks which connect thethermoelement cells in series with one another in the sandwichcomposite.

The two wafers are joined together to form the sandwich composite in themanner already specified further above.

In another preferred embodiment for producing the first and secondconductor tracks and the first and second bodies, a first electricallyconductive layer is applied to the main area of the first substratewafer. That is followed by application of a first layer made ofthermoelectric material, which is of the first conduction type, to thefirst electrically conductive layer.

This first layer made of thermoelectric material is subsequentlypatterned by means of photomask technology and etching in such a waythat a plurality of mutually isolated first bodies are produced on thefirst electrically conductive layer.

After this process step, the first electrically conductive layer ispatterned for example once again by means of photomask technology andetching to form first conductor tracks.

In this case, too, the first electrically conductive layer can, however,also be patterned to form first conductor tracks even before theapplication of the first layer made of thermoelectric material.

Before, during, or after these process steps, a second electricallyconductive layer is applied to the main area of the second substratewafer and a second layer made of thermoelectric material, which is ofthe second conduction type, is deposited on said second electricallyconductive layer.

This second layer made of thermoelectric material is then patterned bymeans of photomask technology and etching in such a way that a pluralityof mutually isolated second bodies are produced on the secondelectrically conductive layer.

The second electrically conductive layer is subsequently patterned toform second conductor tracks for example once again by means ofphotomask technology and etching.

The second electrically conductive layer can also be patterned to formsecond conductor tracks even before the application of the second layermade of thermoelectric material.

The two wafers are joined together to form a sandwich composite withthermoelement cells connected in series in the manner already specifiedfurther above.

In a further preferred embodiment for producing the first and secondconductor tracks and the first and second bodies, a first electricallyconductive layer is again applied to the main area of the firstsubstrate wafer. That is followed by application of a first layer madeof thermoelectric material, which is of the first conduction type, tothe first electrically conductive layer.

A first layer made of thermoelectric material, which is of the firstconduction type, is then deposited onto said first electricallyconductive layer. This first layer is subsequently patterned by means ofphotomask technology and etching in such a way that a plurality of firstbodies are produced on the first electrically conductive layer.

A second layer made of thermoelectric material, which is of the secondconduction type, is then applied to these first bodies and to the freesurface—lying between the first bodies—of the first electricallyconductive layer. This second layer is subsequently patterned once againby means of photomask technology and etching in such a way that aplurality of second bodies are produced on the free surface of the firstelectrically conductive layer.

The first electrically conductive layer can be patterned to form firstconductor tracks before or after the application of the first and secondlayer made of thermoelectric material.

Before, during or after these process steps, a second electricallyconductive layer is applied to the main area of the second substratewafer and is subsequently patterned to form second conductor tracks.

The two wafers are joined together to form a sandwich composite withthermoelement cells connected in series in this case once again in themanner already specified further above.

The particular advantage of the abovementioned methods is that processesof semiconductor technology can be used for fabricating the first andsecond conductor tracks and the first and second bodies. As a result,both the integration level of the thermoelement cells and thefabrication costs for thermoelectric converters can be considerablyreduced. This last is due to the fact that conventional and establishedprocesses of semiconductor technology which are used for the massproduction of semiconductor chips are employed.

The thermoelectric converters fabricated by the method according to theinvention can advantageously be integrated in a simple manner togetherwith elements of microelectronics and/or microsystems technology on oneand the same chip.

With the method according to the invention, the first and the secondbodies can be fabricated in a simple manner from multilayer systemscomprising a multiplicity of thin layers having a different materialcomposition. As a result, the performance of thermoelectric converterscan advantageously be increased by the use of layer sequences that areexactly coordinated with one another.

Three exemplary embodiments of the invention are explained below inconjunction with FIGS. 1a to 4 c, in which:

FIGS. 1a to 1 g show a diagrammatic illustration of the method sequencein accordance with a first exemplary embodiment,

FIGS. 2a to 2 g show a diagrammatic illustration of the method sequencein accordance with a second exemplary embodiment,

FIGS. 3a to 3 g show a diagrammatic illustration of the method sequencein accordance with a third exemplary embodiment, and

FIGS. 4a to 4 b show three-dimensional illustrations of a thermoelectricconverter fabricated according to one of the exemplary embodiments.

In the figures, identical or identically acting constituents are eachprovided with the same reference symbols.

In the exemplary embodiment illustrated in FIGS. 1a to 1 g, firstly afirst electrically conductive layer 10 is fabricated on a main area 8 ofa first substrate wafer 1. Said layer comprises e.g. a metal layer, ametal layer sequence or a highly doped and hence highly conductivesemiconductor layer (e.g. diffused silicon).

The first substrate wafer 1 has overall a low electrical conductivityand is composed, for example, of semi-insulating silicon or has anelectrically insulating layer 14 (e.g. an Si oxide or Si nitride layer)on the side of the main area 8.

On the first electrically conductive layer 10 there is deposited a layer11 made of thermoelectric material (e.g. Bi₂Te₃, Bi₂Se₃, PbTe, Si, Ge,etc.) (FIG. 1a), in which a plurality of doped regions 40 of a firstconduction type (e.g. p-conducting) and a plurality of doped regions 50of a second conduction type (n-conducting) are subsequently formed bymeans of photomask technology and diffusion (FIG. 1b).

The layer 11 with the doped regions 40, 50 is then patterned to formfirst 4 and second bodies 5 by means of one or more conventionalphotomask and etching processes known from semiconductor technology(FIG. 1c).

Afterwards, the first electrically conductive layer 10 is likewisepatterned by means of photomask technology and etching in such a waythat a plurality of mutually isolated thermoelement cells are producedon the first substrate wafer 1, which cells each have a first body 4 andsecond body 5 and a first electrical conductor track 3 connecting saidbodies (FIG. 1d).

As an alternative to the procedure described above, the firstelectrically conductive layer 10 can also be patterned prior to theapplication of the layer 11 made of thermoelectric material.

Furthermore, a metallization layer 13, composed e.g. of a solder (e.g.AuSn) or of gold, is in each case applied to those sides of the firstand second bodies 4, 5 which are opposite to the first conductor track 3(FIG. 1d).

Before, during or after this first wafer process, a second electricallyconductive layer 12 is formed (FIG. 1e) on a main area 9 of a secondsubstrate wafer 2 and patterned (FIG. 1f) to form second electricalconductor tracks 6.

Analogously to the substrate wafer 1, the second substrate wafer 2 hasoverall a low electrical conductivity and is composed, for example, ofsemi-insulating silicon or the substrate wafer 2 has an electricallyinsulating layer 15 (e.g. an Si oxide or Si nitride layer) on the sideof the main area 9.

After the patterning of the second conductor tracks 6, the secondsubstrate wafer 2 is placed with the latter onto the metallizationlayers 13 of the first and second bodies 4, 5 and aligned in such a waythat the second conductor tracks 6 bear on the metallization layers 13of the first and second bodies 4, 5 and connect the previously formedpairs comprising respectively a first and a second body 4, 5 in serieswith one another (FIG. 1g).

The second electrical conductor tracks 6 and the metallization layers 13are subsequently connected to one another, for example by means ofsoldering, adhesive bonding or thermocompression.

This sandwich composite comprising the two substrate wafers 1, 2 and theintervening thermoelement cells is then separated to form a plurality ofthermoelectric converters, e.g. by sawing. A plurality of different oridentical thermoelectric converters can be produced from a sandwichcomposite. The respective first and last thermoelement cell of a seriescircuit of thermoelement cells of a thermoelectric converter each has anelectrical pad 30, 31, via which the thermoelectric converter can beelectrically connected, for example by means of bonding wires 32, 33.

However, the thermoelectric converters can also readily be embodied asSMD (Surface Mount Device) components, by the electrical pads 30, 31being routed in an electrically conductive manner to that side of thefirst substrate wafer 1 which is opposite to the main area 8.

An advantageous process in the separation of the sandwich composite is atwo-part sawing process in which one of the two substrate wafers 1, 2 ismade smaller than the other. This facilitates the electrical connectionof the thermoelectric converter (cf. FIG. 1g).

As an alternative to the above-described method (whole-area coating ofthe substrate wafers and subsequent patterning), the conductor tracks 3,6 can be fabricated in a directly patterned manner by means of maskingand metallization or diffusion of dopant into the respective substratewafer 1, 2.

The exemplary embodiment illustrated in FIGS. 2a to 2 g differs fromthat mentioned above essentially by the fact that the following methodsare carried out in order to fabricate the first 4 and second bodies 5:

after the formation of the first electrically conductive layer 10, afirst layer 20 made of thermoelectric material, which has the firstconduction type, is deposited onto said first electrically conductivelayer (FIGS. 2a and 2 b);

the first layer 20 is patterned by means of photomask technology andetching in such a way that a plurality of mutually isolated bodies 4 areproduced on the first electrically conductive layer 10 (FIG. 2c);

a second layer made of thermoelectric material 21, which has the secondconduction type, is applied to that surface of the first electricallyconductive layer 10 which is uncovered after the patterning of the firstlayer 20 between the first bodies 4, and to the first bodies 4 (FIG.2d); and

the second layer made of thermoelectric material 21 is patterned bymeans of photomask technology and etching in such a way that a pluralityof second bodies 5 are produced on the first electrical layer 10 betweenthe first bodies 4 (FIG. 2e).

The first electrically conductive layer 10 can be patterned to formfirst electrical conductor tracks 3 before or after the application ofthe first 20 and second layer 21 made of thermoelectric material.

The fabrication of the second substrate wafer 2 with the secondelectrical conductor tracks 6 and the connection of the two wafers areeffected analogously to the first exemplary embodiment (FIGS. 2f and 2g).

In the exemplary embodiment illustrated in FIGS. 3a to 3 f, the methodfor fabricating the first and second bodies 4, 5 differs from that ofFIGS. 2a to 2 e essentially by the fact that the second layer 21 made ofthermoelectric material, which has the second conduction type, is notdeposited on the first substrate wafer 1. In this case, after theformation of the second electrically conductive layer 12 on the mainarea 9 of the second substrate wafer 2, the second layer made ofthermoelectric material 21 is deposited thereon and subsequentlypatterned by means of photomask technology and etching in such a waythat the second bodies 5 are produced on the second electricallyconductive layer 12.

In the exemplary embodiments of FIGS. 1a to 1 g and of FIGS. 3a to 3 e,as an alternative to the method by means of deposition, the layer 11made of thermoelectric material or the first 20 and second layer 21 madeof thermoelectric material can be fabricated as wafers which areconnected to the first substrate wafer 1 or respectively to the first 1and respectively the second substrate wafer 2 by means of wafer bonding.Tungsten silicide can be used as boding and metallization material (forthe electrical conductor tracks).

FIGS. 4a and 4 b show three-dimensional illustrations of athermoelectric converter fabricated according to one of the methodsdescribed above. FIG. 4a is an exploded illustration and in FIG. 4b thesecond substrate wafer 2 is shown transparent.

In the exemplary embodiments described above, the thickness of thesubstrate wafers 1, 2 is advantageously 100-300 μm. The thickness of thelayers 11, 20, 21 made of thermoelectric material is preferablyapproximately 50 μm. The thickness of the electrically conductive layers10, 12 is about 1 μm.

What is claimed is:
 1. A method for fabricating a thermoelectricconverter having a plurality of series-connected thermoelement cells,each of which has a first body made of thermoelectric material of afirst conduction type that is connected by a first electrical conductortrack to a second body made of thermoelectric material of a secondconduction type, where the thermoelement cells are connected to oneanother in series by a second electrical conductor track and arearranged in a sandwich-like manner between a first substrate wafer and asecond substrate wafer, the method comprising: forming a firstelectrically conductive layer on a main area of the first substratewafer; applying a first layer made of thermoelectric material, which isof the first conduction type, to the first electrically conductivelayer; patterning the first layer made of thermoelectric material toform the first bodies; patterning the first electrically conductivelayer to form the first conductor tracks; forming a second electricallyconductive layer on a main area of the second substrate wafer; applyinga second layer made of thermoelectric material which is of the secondconduction type to the second electrically conductive layer; patterningthe second layer made of thermoelectric material to form the secondbodies; patterning the second electrically conductive layer to form thesecond conductor tracks; and connecting the first substrate wafer andthe second substrate wafer such that the first and second electricalconductor tracks and the first and second bodies are arranged betweenthe first and second substrate wafers to form the plurality ofseries-connected thermoelement cells.
 2. The method as claimed in claim1, in which, prior to the application of the second layer made ofthermoelectric material, the second electrically conductive layer ispatterned to form second conductor tracks.
 3. The method as claimed inclaim 1, in which, prior to the application of the first layer made ofthermoelectric material, the first electrically conductive layer ispatterned to form first conductor tracks.
 4. The method as claimed inclaim 1, in which, prior to the application of the first layer made ofthermoelectric material, the first electrically conductive layer ispatterned to form first conductor tracks and, prior to the applicationof the second layer made of thermoelectric material, the secondelectrically conductive layer is patterned to form second conductortracks.
 5. The method as claimed in claim 1, in which the first layerand the second layer made of thermoelectric material are fabricated aswafers which are connected to the first substrate wafer and the secondsubstrate wafer, respectively, by means of wafer bonding.
 6. The methodas claimed in claim 1, in which Si wafers are used as the substratewafers.
 7. The method as claimed in claim 1, in which the first and thesecond bodies are fabricated from multilayer systems comprising amultiplicity of layers having a different material composition.
 8. Themethod as claimed in claim 1, in which electrically insulating wafersare used as the substrate wafers.
 9. The method as claimed in claim 1,in which wafers having an electrically insulating layer are used as thesubstrate wafers.